1. Field of the Disclosure
The disclosure relates to high power fiber laser systems. Particularly, the disclosure relates to a high power low-loss low-mode (“LM”) fiber system emitting a kW level, high quality laser output and configured with a cladding mode absorber which is operative to efficiently filter out unwanted forward propagating and backreflected cladding light.
2. Prior Art
High power kW-level fiber laser systems are used in a growing number of applications. As fiber lasers mature towards commercial deployment, an intense focus on their power, quality and reliability and that of their components is required. With the current progress in this field, reliability requirements are made at increasingly higher power levels reaching the multi-kilowatt levels. To enable the desired power levels, multiple single mode (“SM”) fiber laser systems are optically and mechanically coupled together in a combiner. To efficiently perform at kW power levels and emit an output, having a low mode (“LM”) output, the combiner should successfully deal with a the structural difficulties some of which, such as mechanical coupling of fibers and power loses in forward and backreflection directions, are of a particular interest here.
In general, a process of fabricating high power combiner includes fusing aligned output fibers of respective fiber lasers/amplifiers in a bundle, tapering the latter, cleaving and splicing the tapered bundle to a system output delivery fiber. The fabrication of the combiner which initially assumes a bow-tie configuration, whose waist is further severed in half, may result in structural defects (burrs) of cladding which further, during the combiner's deployment, may detrimentally affect the quality of the output laser beam and power thereof
With outputs of fiber laser systems reaching several kWs, forward propagating core-guide light tend to bleed into a cladding as it propagates through and air-quartz interfaces. Once in the cladding, high power signal light induces thermal loads on a polymeric coating surrounding the cladding and shielding the fiber from mechanical loads.
Accordingly, a need exists in a device capable of efficiently dealing with undesirable consequences of forward propagating signal light decoupled from the core.
In addition to forward propagating signal light, light reflected from the surface to be laser treated and hence further referred to as a backreflected light is also damaging both to a combiner itself and to system components upstream therefrom. Similar to forward propagating light, backreflected light has to be removed from a waveguide before it propagates back into individual fiber laser systems.
In summary, to attain an optical power of up to kWs at the output of the LM waveguide, the combiner needs a special configuration which would allow the following:                (1) Reliably fix input fibers together without degrading the quality of the output beam;        (2) Efficiently distribute and utilize power losses of forward and backward propagating light; and        (3) Provide protection of the fibers from environmental impurities as a result of heat-induced deformation.        
A further need exists for a high power fiber laser system based on the above discussed SM-LM combiner and capable of emitting a high quality product parameter beam reaching several kWs.
A need therefore, exists for a LM high power fiber laser system provided with a clad absorber which is configured to meet the above-listed conditions.